In wireless networks, a handoff usually occurs when a user of a client moves from a current cell to another cell. Examples of the client include a laptop, a personal digital assistant (PDA), a mobile phone, and the like. Each cell is serviced by at least one access point. Handoff refers to the process of transferring an ongoing call or data session from a channel connected to the current cell to a channel connected to another cell. It is desirable to have low interference and a smooth transfer during the handoff. In order to achieve this, the time span of the handoff should be minimized.
The handoff process involves scanning access points that are adjacent to a first access point, while the client is associated with the first access point. This scanning involves analyzing the beacon frames broadcast by each access point adjacent to the first access point. These beacon frames are broadcast regularly by all the access points. The time at which an access point broadcasts the beacon frame is referred to as the target beacon transmission time (TBTT). Further, each beacon frame is broadcast after regular intervals called beacon intervals. Consider a case of a handoff of the client to a second access point. The scanning process can be hastened if the client is aware of the next target beacon transmission time of the second access point. The current TBTT and the beacon interval can be used to predict the next TBTT.
The scanning is followed by the authentication of the client by the second access point. The authentication server authenticates the client. This authentication is usually performed by using an Authentication, Authorizing and Accounting (AAA) protocol such as, for instance, Remote Authentication Dial-In Service (also known as the RADIUS protocol) as currently defined in Internet Engineering Task Force (IETF) Request for Comments (RFCs) 2865 and 2866. This is followed by a 4-way handshake between the client and the second access point.
Two known methods exist for collecting the TBTT and the beacon interval of access points of interest. The first method is mentioned above, which is by scanning the beacon frame of each access point adjacent to the first access point separately. However, a particular shortcoming of this method is that often, some of the access points adjacent to the first access point are hidden. One reason for an access point being hidden is the presence of an intercepting infrastructure such as a building. In such a scenario, the client is not able to receive the beacon frames of the hidden access points and is, therefore, unable to scan such beacon frames to determine the TBTT and beacon interval for the hidden access point.
The other method for learning the TBTT and the beacon interval is by using Institute of Electrical and Electronics Engineers (IEEE) standard 802.11k neighbor maps. Neighbor maps comprise a list of access points adjacent to the first access point, and their target beacon transmission times and beacon intervals. However, a shortcoming of this method is that it does not account for the effects of clock drifting. Clock drifting causes the TBTTs and the beacon intervals to change, thereby, making it difficult to determine whether the TBTTs and the beacon intervals recited in the neighbor maps are current.
In light of the above discussion, there is a need for a method for learning the TBTT and the beacon interval more efficiently.